In motor vehicle construction it has been common for some time to integrate control devices for the engine or transmission into the motor vehicle assembly which is to be controlled, i.e. the engine or transmission. It is in particular the transmission control devices which, as a so called in-situ control device, form an extremely compact unit. Compared with the traditional use of external add-on control devices, this arrangement has enormous advantages as regards quality, costs, weight and functionality. The result is in particular a significant reduction of plug connections and lines which may be susceptible to failure.
If the control device is to be integrated into the transmission, it has to fulfil high requirements with regard to its thermal and mechanical load capacity. Functionality has to be ensured for both a broad temperature range (approximately −40° C. up to 200° C.) and mechanical vibrations (up to approximately 40 g). Moreover, as the control device is surrounded by aggressive media such as gear oil, it has to be oil-tight.
However, in in-situ transmission control devices, the electronic functional components, such as the microcontroller, or bonding pads or bonding wires in the circuit carrier interior space can still be damaged as a result of the unwanted entry of, for example, sulphurous pollution gases due to diffusion from the outside through the seals into the circuit carrier interior space of the control device housing or by gas evolution, for example of sealing materials, the housing or plastic materials. This may potentially cause the control devices to fail completely much sooner than their predicted lifetime expectancy.
In particular, if metal-containing components in the interior space of transmission control devices come into contact with corrosive media such as sulphurous gases, water or moist air, said media attack the metal, leading to its corrosion. It is in particular the oxygen dissolved in water which reacts with the metal. Electrons are withdrawn from the metal, and the positively charged ions can enter into solution.
One way to prevent this is the so-called cathodic corrosion protection. A so-called sacrificial anode is conductively connected to the metal to be protected. The metal to be protected is the cathode, and the less noble metal is the sacrificial anode. The result is a current flowing towards the metal to be protected. The electrons are now withdrawn from the less noble metal. The transport of the charged particles occurs through the direct contact of the two metals or through water or water vapor as an electrolyte.
This method for preventing the corrosion of metal-containing components in the interior space of transmission control devices can not be applied here, as it would lead to undefined leakage currents or even to short circuits.
Another way to prevent such corrosion is described in DE 34 42 132 C2. In a housing, an encapsulated microelectronic element is enclosed by silicone rubber wherein a getter material is dispersed as an ultrafine-grained powder containing a barium-aluminum alloy. This powder, however, is suitable only to a limited extent to bind in particular aggressive gaseous components such as sulphur. Rather, it has shown that silicone rubber has the effect of a sponge, in particular for sulphurous gases, and thus even increases corrosion.
Another way to prevent such corrosion is described in DE 39 13 066 A1. A recess for receiving getter material is formed in the cover of a housing for electronic components. The getter material is covered and fixed by a foil. Immediately before the housing is assembled, the foil is perforated such that the getter material can fulfil its function. This method for preventing corrosion is very complex.